I have a medium format camera with a multiple exposure switch. This arms the shutter without advancing the film so that multiple shots can be taken on the same physical film section. This is a fairly common feature among any non-entry-level film SLRs. On 35mm cameras you can load a roll of film and mark with a permanent marker the point that the leader enters the takeup spool. You can then shoot a whole roll of 'backgrounds', hit the rewind switch, go somewhere else, re-load the film so that the pen mark is lined up, and shoot the whole roll of film for second exposures. This technique is useful when you e.g. don't have a nice enough day / subject for the final multiple exposure photo but you are in a place that would provide interesting backgrounds.
None of the medium format cameras I've used have the ability to rewind the film. Once the film is moved over to the take-up spool it is inverted - the film at the beginning is now on the inside of the roll. The beginning of the film has a little arrow indicating where you should wind to but this doesn't exist at the tail end because it doesn't make sense.
Is re-loading an exposed medium format roll of film, re-shooting the whole thing with the lens cap on (possibly in a dark room), then re-loading it into the camera a safe thing to do? Is there a better way?
